Wars used to be justified by their goals. Territorial expansion, restitution or claiming property, securing a throne, and mass religious conversion were all acceptable goals under the tenets of Just War Theory; and they all justified war to begin with, particularly if you ended up attaining them at the end. But since we no longer live in a legal world that sees war as the continuation of diplomacy, when we do end up going to war, it is becoming harder to define our goals. Indeed, our preoccupation with the legality or the justification for war often clouds our ability to see war – and judge it – in terms of goals sought and achieved.

The modern jus ad bellum has aimed to banish war as a legal regime: From an acceptable instrument of self-interest, enforcement and punishment, war has become an evil of last resort, and (absent a U.N. Security Council authorization), carried out in the name of self-defense only. In The Internationalists, Oona Hathaway and Scott Shapiro trace the transformative moment to the 1928 Kellogg-Briand Pact, urging us, through a compelling history-of-ideas narrative, to celebrate the Pact’s prohibition on wars as nothing short of revolutionary. Contrary to much conventional thought that dismisses the Pact as an ironic prelude to the worst war in human history, Hathaway and Shapiro see the Pact as “one of the most transformative events of human history” (p. xiii), not only for its normative innovation, but also for its practical role in driving the decline in interstate wars since the second half of the twentieth century.

Whether the watershed moment was in fact 1928, Hathaway and Shapiro are undoubtedly correct to identify – and celebrate – the normative effort to restrict wars to those carried out in self-defense. (The authors are equivocal about the Pact’s tacit allowance for such wars, as opposed to the U.N. Charter’s explicit authorization. However, I know of no recitation of the customary right of defensive wars that denies this right on the basis of the 1928 Pact).

But how much does the defensive paradigm actually restrict nations? As Hathaway and Shapiro acknowledge, self-defense, and its expanded notions that include, inter alia, defense of others, is a highly malleable concept. Indeed, they lament towards the end of the book, “…the growing reliance upon self-defense as a justification for using force… threatens to make self-defense the exception that swallows the rule against war.” Some of the malleability is inherent in any standard. Some is the inevitable result of the fact that today’s world exposes countries and citizens to a wider array of potential threats from a wider array of actors, than ever before. This might help explain, in part, why alongside the decline in traditional interstate wars, the incidence of transnational uses of force is on the rise. And it is not at all clear that such transnational uses of force would not pass muster under the law; or, indeed, what the muster actually requires.

There is an even bigger problem with the growing resort to defensive wars. Restricting force to instances of self-defense not only permits recourse to war in those loosely defined instances. It also limits our ability to identify a clear moment of victory when the goals of war are satisfied and the war has no further justifiable ground upon which to continue.As I’ve suggested elsewhere, the problem lies in the effects that this defensive paradigm has on the goals that states are pursuing. What are permissible goals for wars in self-defense? In a self-help world, rife with threats, how do we know we are indeed defended? And how safe can we seek to be as a matter of law?

The broad list of affirmative goals, resting on a broad list of permissible justifications, that warring parties were free to pursue in the pre-Pact era was, to modern eyes, wildly permissive. But in another sense it was also usefully restrictive. For once set, the asserted goals gave warring parties clear yardsticks by which to evaluate the success of the war they were undertaking. And this, in turn, indicated when the war had to end. A war for the collection of debt had to end, as a conceptual matter, once the debt had been collected. A war for territory should have ended once the territory was captured. And pitched battles would, by sundown, determine who would inherit the throne. Even just punishment was bound by what retribution and deterrence could reasonably allow.

When all war goals must be couched in terms of self-defense, we are left with far fewer metrics by which to evaluate when the defensive need has been met. Defense might not require conquest. But just as formal conquest in terms of taking and annexing territory has virtually disappeared, as Hathaway and Shapiro tell us, invading and controlling territory is ubiquitous. And such campaigns can hardly be rejected categorically as illegal. For the self-defense paradigm does not tell us how much territory can be captured, for how long it can be occupied, or the terms under which the occupation must end.

Take, for example, the wars in Afghanistan and Iraq. One could debate whether a just cause of self-defense existed in either case. But let us assume, for the sake of argument, that it did in both. We would easily say that it would be impermissible to annex territory, enslave the population, plunder resources, or convert religious faith. These, thanks to Kellogg-Briand and the U.N. Charter, would be clearly impermissible goals. We would be harder pressed to define the permissible goals for these wars: Regime change? Establishing democracy and the rule of law? Protecting minorities? All these, as well as economic development, child literacy, or gender equality have been publicly declared by relevant actors as “metrics of success” in these theaters, as proof of our growing safety. Add to the mix the additional development in law where self-defense is understood under certain conditions to encompass the defense of others. The goal of promoting the welfare of Afghans and Iraqis can now be justified, in principle, not only on the grounds of American security interests, but also as an ostensibly legitimate interest in the security of Afghans and Iraqis.

The result is that under our contemporary self-defense paradigm, there is no recognizable moment in which the war has achieved its legitimate goals. Victory cannot be measured by concrete benefits but only by the absence of concrete harms. An absence is a notoriously hard thing to prove, especially an absence or cessation of threat. Modern wars, as a consequence, may or may not be founded in more morally legitimate reasons than their predecessors. At the same time, they are more amorphous, and more difficult to judge and to restrict. In some sense, they not only tolerate – but invite – endless wars.

Just War Theory demanded declarations of war, as Hathaway and Shapiro document both in their book and in this important compilation. When justifications and goals were so intertwined, there was no need to specify at great length the goals of any war, even though in some cases, such pronouncements did exist. Today, we demand only the justification of war on the basis of Article 51 of the UN Charter – the self-defense exception – but not the clear articulation of what the war is designed to achieve. Countries’ statements on the basis of Article 51, such as those lodged by countries fighting ISIS in Syria, tend to be short, general, and focus only on the self-defense justification. Even contemporary debates within the United States over an updated AUMF center on authority and justification, without offering a clear statement of goals. Perhaps we – international lawyers and publics at large – should demand that when our leaders take us to war, they give us a clear and detailed articulation of goals, which can be debated and evaluated, to ensure that the progressive move towards the limitation of war continues.

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About the Author(s)

Rita E. Hauser Professor of Human Rights and Humanitarian Law at Harvard Law School, Faculty Director of the Program on International Law and Armed Conflict (PILAC), Member of the Program on Negotiation Executive Board